The international PCT patent application WO2013135861A2 published 19 Sep. 2013 by Terje Sira and Tor Bjørnstad presents an apparatus for tracer based flow measurement. The apparatus comprises a tracer chamber for installation on a production tubing. The tracer chamber is arranged for holding tracer and is arranged to be linked, in use, to the pressure in an annulus about the production tubing. The tracer chamber comprises an outlet for fluid communication between the tracer chamber and the fluid within the production tubing. Tracer is released through the outlet into the production tubing in accordance with a pressure differential between the annulus and the production tubing. The general principle of Sira and Bjørnstads published application is illustrated in FIG. 1 of WO2013135861A2. Its tracer chamber is arranged in a geological formation made of a hydrocarbon production zone, such as sandstone or carbonates, framed by impermeable layers above and below, such as shales or salts. The tubing has been installed in the formation and it is separated from the geological rocks by a sand-filled annulus and a casing or a naturally cut borehole wall. In the annulus the production zone is typically isolated from the geological formations above and below by packers. The production from the zone is controlled at the level of one or more ICD's.